Manipulation of Ascorbate Biosynthetic, Recycling, and Regulatory Pathways for Improved Abiotic Stress Tolerance in Plants

Abstract

Abiotic stresses, such as drought, salinity, and extreme temperatures, are major limiting factors in global crop productivity and are predicted to be exacerbated by climate change. The overproduction of reactive oxygen species (ROS) is a common consequence of many abiotic stresses. Ascorbate, also known as vitamin C, is the most abundant water-soluble antioxidant in plant cells and can combat oxidative stress directly as a ROS scavenger, or through the ascorbate-glutathione cycle-a major antioxidant system in plant cells. Engineering crops with enhanced ascorbate concentrations therefore has the potential to promote broad abiotic stress tolerance. Three distinct strategies have been utilized to increase ascorbate concentrations in plants: (i) increased biosynthesis, (ii) enhanced recycling, or (iii) modulating regulatory factors. Here, we review the genetic pathways underlying ascorbate biosynthesis, recycling, and regulation in plants, including a summary of all metabolic engineering strategies utilized to date to increase ascorbate concentrations in model and crop species. We then highlight transgene-free strategies utilizing genome editing tools to increase ascorbate concentrations in crops, such as editing the highly conserved upstream open reading frame that controls translation of the GDP-L-galactose phosphorylase gene.

Keywords: antioxidant; ascorbic acid; biosynthesis; genetic engineering; genetic modification; genome editing; recycling; regulation; vitamin c.

Figure 1

The ascorbate–glutathione cycle—a major antioxidant system of plants cells. In this cycle, electrons flow from NADPH to H₂O₂. Dashed arrows indicate non-enzymatic disproportionation. APX, ascorbate peroxidase; MDAR, monodehydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione reductase.

Figure 2

The four proposed ascorbate biosynthetic pathways in higher plants—the L-galactose, L-gulose, myo-inositol, and D-galacturonate pathways. Dashed arrows indicate multiple and/or unknown biosynthetic steps in higher plants. PMI, phosphomannose isomerase; PMM, phosphomannose mutase; GMP, GDP-D-mannose pyrophosphorylase; GME, GDP-D-mannose-3′,5′-epimerase; GGP, GDP-L-galactose phosphorylase; GPP, L-galactose-1-P phosphatase; L-GalDH, L-galactose dehydrogenase; L-GalLDH, L-GalLDH L-galactono-1,4-lactone dehydrogenase; L-GulLO, L-gulono-1,4-lactone oxidase; MIOX, myo-inositol oxygenase; D-GalUR, D-galacturonate reductase.